The present invention relates to a method for casting of metals, and in particular to a method for continuous or semi-continuous casting of a strand in a mold, wherein the flow of metal in non-solidified parts of the cast strand is acted on and controlled by at least one static or periodically low-frequency magnetic field applied to act upon the molten metal in the mold during casting. The present invention also relates to a device for carrying out the invented method.
In a process for continuous or semi-continuous casting, a metallic melt is chilled and formed into an elongated strand. Depending on its cross-section dimensions, the strand is called a billet, a bloom or a slab. During casting a primary flow of hot metal is supplied to a chilled mold wherein the metal is cooled and at least partly solidified into an elongated strand. The cooled and partly solidified strand leaves the mold continuously. At the point where the strand leaves the mold, it includes at least a mechanically self-supporting skin surrounding a non-solidified center. The chilled mold is open at both of its ends in the casting direction and is preferably associated with means for supporting the mold and means for supplying coolant to the mold and the support. The chilled mold preferably includes four mold plates, preferably made of copper or other material with a suitable heat conductivity. The support means are preferably beams with internal channels for supply of coolant, normally water, thus such support beams are often called water beams. The water beams are arranged around and in good thermal contact with the chilled mold to fulfill its double function of supporting and cooling the mold.
The hot primary metal flow is supplied either through a nozzle submerged in the melt (closed casting), or through a free tapping jet (open casting). These two alternative methods create separate flow situations and effects how and where the magnetic field(s) is/are applied. If the hot primary metal flow is allowed to enter the mold in an uncontrolled manner, it will penetrate deep in the cast strand, which is likely to negatively effect its quality and productivity. Non-metallic particles and/or gas might be drawn in and entrapped in the solidified strand. An uncontrolled hot metal flow in the strand might also cause flaws in the internal structure of the cast strand. Also a deep penetration of the hot primary flow might cause a partial remelt of the solidified skin, such that melt penetrates the skin beneath the mold, causing severe disturbance and long down-time for repair. To avoid or minimize these problems and improve the production conditions, according to the disclosure in European Patent Document EP-A1-0 040 383, one or more static magnetic fields can be applied to act on the incoming primary flow of hot melt in the mold to brake the incoming flow and split it up to create a controlled secondary flow in the molten parts of the strand. The magnetic field is applied by a magnetic brake which includes one or more magnets. Favorably, an electromagnetic device, i.e., a device comprising one or more windings such as a multi-turn coil wound around a magnetic core, are used. Such an electromagnetic brake device is called an EMBR.
According to the disclosure in European Patent document EP-B1-0 401 504, magnetic fields are applied in two levels, arranged one after the other in the casting direction, during casting with a submerged entry nozzle (closed casting). The magnets include poles having a magnetic band area covering essentially the whole width of the cast strand and one first level is arranged above and one second level below the outlet ports of the submerged nozzle. Further, EP-B1-0 401 504 teaches that the magnetic flux should be adopted to the casting conditions, i.e., the strand or mold dimensions and casting speed. The magnetic flux and the magnetic flux distribution are adopted to ensure a sufficient heat transport to the meniscus to avoid freezing while at the same time the flow velocity at the meniscus is limited and controlled so that the removal of gas or inclusions from the melt is not put at risk. A high uncontrolled flow velocity at the meniscus might also cause mold powder to be drawn down into the melt. It is also suggested in this document that an optimum range exists for the flow velocity at the meniscus, see FIG. 9. It is suggested that the magnetic flux density over the mold is adopted before a casting operation based on the specific conditions assumed to prevail during the coming cast operation. To accomplish this EP-B1-0 401 504 suggests a mechanical magnetic flux-controlling device which is arranged to move the magnetic poles in essentially their axial direction to change the distance between the poles comprised in one cooperation pair and arranged facing each other on opposite sides of the mold, see FIG. 15 and column 8, lines 34 to 50. Such a mechanical magnetic flux-controlling device must however be extremely rigid to accomplish a stable magnetic flux density, especially when subject to the large magnetic forces prevailing under operation of the brake while at the same time being capable of small movements to accomplish the adjusting changes in flux density required as the flux density has a high sensitivity to changes in the distance between the poles. Such a mechanical magnetic flux density-controlling device requires a combination of heavy gauge material, rigid construction and small movements in the direction of the magnetically field, which will be hard and costly to accomplish. According to one alternative embodiment the mechanical flux density device is formed by partial substitution of the poles by non-magnetic material such as stainless steel, i.e., by a change in the configuration of the poles and thereby an alteration of the pattern of the magnetic flux in the mold before each cast. Similar ideas as to the configuration of the poles, are discussed in other documents such as EP-Al-577 831 and WO 92/12814. The patent document WO 96/26029 teaches the application of magnetic fields in further levels including one or more levels at or just downstream of the exit end of the mold to further improve the control of the secondary flow in the mold. Flux density-controlling devices of these types based on reconfiguration and/or movements of the poles by mechanical means must be complemented with means for securing the magnet core or partial cores to withstand the magnetic forces and is thus intended for presenting the magnetic flux density and adopted to casting conditions predicted to prevail during a forthcoming casting, and it will include costly and elaborative development work to use such devices for on-line regulation of the magnetic flux density.
According to European Patent Document EP-A1-0 707 909, the flow velocity at the meniscus shall be set within a range of 0.20-0.40 m/sec for a continuous casting method wherein a primary flow is supplied to mold through a nozzle capable of controlling the incoming flow and wherein a static magnetic field having a substantially uniform magnetic flux density distribution over the whole width of the mold is applied to act on the metal in the mold. It further teaches that the flow at the meniscus can be held within this range by setting several parameters such as;
the angle of the port(s) in the submerged nozzle;
the position of the nozzle ports) within the mold;
the position of the magnetic field; and
the magnetic flux density.
The angle and position of the nozzle port(s) as well as the position of the magnetic field(s) are determined and preset before the start of casting and the magnetic flux is controlled according to one out of two different algorithms. The choice of algorithm to be used is dependent on the position of the magnetic field relative the primary flow, i.e., if the primary flow out of the nozzle port(s) traverses a magnetic brake field or not before hitting the side-wall. The algorithms) are based on one measured value only, the flow velocity at the meniscus when no magnetic field is applied, i.e., a historical value measured at an earlier casting or possible at the start of the casting if the casting are started with the brake off. The other values of the algorithms are all preset. The values included are the mold width and thickness which truly is constant and the average flow velocity of molten steel through the nozzle port(s), i.e., the primary flow, which is treated as a constant value or possibly as a predetermined function of time. Thus will in fact the magnetic flux density also according to this method be preset as it will be based on predetermined and preset parameters only and the control will not account for any change in the actual casting conditions or a dynamically progressing process and will consequently not be capable of adjusting the flux density on-line based on a change in the actual flow. Examples of parameters or conditions which effect the secondary flow and are likely to change during casting are the ferrostatic pressure at the nozzle port(s), nozzle angle(s) or nozzle dimensions due to erosion or clogging, the superheat in the primary flow, i.e., its temperature relative the melting point, chill at meniscus, and level of meniscus in mold. The primary flow might also have to be adopted due to a change in casting speed or other separately controlled production parameter.
It is a primary object of the present invention to provide a method for continuous casting of metal wherein the flow in the mold is controlled during casting by an on-line regulation of the magnetic flux density of a magnetic field applied to act on the metal to brake and split the incoming primary flow of hot metal and form a controlled secondary flow pattern in the mold. The on-line regulation shall be provided throughout essentially the whole casting and be based on the actual casting conditions or operating parameters prevailing in the mold or effecting the conditions in the mold at that moment to provide a cast product with a minimum of defects produced at the same or improved productivity.
As the flow at the meniscus has been shown critical for both removal of impurities, trapping of mold powder and gas and indicative of the flow situation prevailing in the mold, it is also an object of the present invention to monitor the flow at the meniscus throughout the casting by direct or indirect methods and include any change detected in this flow in the online regulating of the magnetic flux density to ensure a minimum of trapping or accumulation of non-metallic inclusions, mold powder or gas in the cast products. It is further an object of the present invention to provide a device for carrying out the invented method.
Other advantages of the present invention will became apparent from the description of the invention and the preferred embodiments of the invention, including its capabilities to provide an improved and controlled flow pattern throughout the casting also when one or more parameters change and the thereby increased capability to, over a wide range of operating parameters, mold dimensions, metal compositions, etc., control the solidification conditions in the cast product, conditions for removal of non-metallic impurities from the cast product and the entrapment of mold powder or gas in the cast products, so that even when one or more of these parameters changes for whatever reason during casting, the casting conditions can remain essentially stable or be adjusted to be within preferred limits.
To achieve this in the inventive casting method (continuous or semi-continuous), a primary flow of hot metallic melt is supplied into a mold and at least one static or periodically low-frequency magnetic field is applied to act on the melt in the mold. One or more magnetic fields are arranged to brake and split the primary flow and form a controlled secondary flow pattern in the non-solidified parts of the cast strand. To achieve the desired secondary flow the magnetic flux density of the magnetic field is regulated based on casting conditions. To accomplish the primary object of the invention, the secondary flow in the mold is monitored throughout the casting and any detected change in the monitored flow is fed into a control unit where the change is evaluated. The magnetic flux density is thereafter regulated based on this evaluation to maintain or adjust the controlled secondary flow. Preferably the flow velocity of the secondary flow in at least one specific point in the mold is measured continuously throughout essentially the whole casting. As an alternative to the continuous measurement of the flow velocity, the flow velocity can also be discontinuously (intermittently) measured or sampled throughout essentially the whole casting operation. Upon detection of any change in the flow, information on this change will, whether detected by continuously measurement or sampling, be fed into the control unit where it is evaluated. The magnetic flux density is thereafter regulated based on this evaluation.
A device for carrying out the invented method for continuous or semicontinuous casting of metals comprises a mold for forming a cast strand, means for supply of a primary flow of a hot metallic melt to the mold, and magnetic means arranged to apply at least one magnetic field to act upon the metal in the mold and is according to the present invention arranged with the magnetic means associated with a control unit. The control unit is associated with detection means which are arranged to monitor metal flow in the mold and detect any changes in the flow. Upon detection of a change in the casting conditions or in the flow information, the change is fed into the control unit which comprises evaluation means to evaluate the detected change and control means to regulate the magnetic flux density of the magnetic field based on the evaluation of the detected change in the flow.
The detection means can be any known sensor or device for direct or indirect determination of the flow velocity in a hot metallic melt, such as flow sensors based on eddy current technology or comprising a permanent magnet, temperature sensors by which a temperature profile of, e.g., one of the narrow sides or the meniscus can be monitored, a level sensing device for determination and supervision of level height and profile of a melt surface in a mold, the meniscus. Suitable detection means will be exemplified and described in more detail in the following.
The control unit comprises means, preferably in the form of an electronic device with software in the form of a algorithm, statistical model or multivariate data-analysis for processing of casting parameters and information from the detection means on flow, and means for regulating the magnetic flux density based on the result of the processing. According to one embodiment of the invention, the control unit is arranged within a neural network comprising electronic means for supervision and control of further steps and devices associated with the casting operation. The control unit also includes means for the regulation of the magnetic flux density of the magnetic brake. For an electromagnetic brake this is best accomplished by control of the amperage fed to the windings in the electromagnets of the electromagnetic brake. This is accomplished by any current limiting device controlled by an out-signal from the control unit. Alternatively, for an electromagnet which is connected to a voltage source, the voltage can be controlled by the out-signal from the control unit, thus indirectly controlling the amperage of the current in the magnet windings. The control unit will be further exemplified in the following.
As the flow conditions can vary within the mold, has it in some cases been shown desirable to monitor the flow at two or more locations within the mold and also to apply the magnetic fields in such a way that the magnetic flux density of one magnetic field can be regulated separately and independently of any other magnetic fields based on the flow prevailing in the part of the mold on which the magnetic field is applied to act. The typical situation is that for a slab mold wide two wide sides and a tapping point in the center of the mold, at least one magnetic circuit is arranged to apply at least one magnetic to act on the melt in each half of the mold, i.e., the mold is, in the casting direction, split into two control zones, each control zone comprising a half of the mold and is disposed on each side of a plane comprising the center line of the wide sides. The flow at the meniscus is measured directly or indirectly for both control zones, i.e., mold halves, and the left control zone sensor is associated with means for regulating the magnetic flux density of a magnetic field acting on the melt in the left half of the mold and a right control sensor is associated with means for regulating the magnetic flux density of a magnetic field acting on the melt in the right half of the mold. The mold can, naturally, be divided into zones of any number and shapes where at least one sensor and at least one magnetic flux density-regulating means is associated with each zone. Using two control zones ensures that an essentially symmetrical two-loop flow is developed in the upper part of the mold and that the risks of the two-loop flow developing to an unsymmetrical or unbalanced flow showing, e.g., marked differences in the flow velocities at the meniscus for the two mold halves, a so called biased flow, or even in the extreme case transforming into an undesired one-loop flow, where the melt flows up along one molds side, across the meniscus to the other side, down and further back across the mold at level with or just downstream the nozzle ports, is essentially eliminated.
According to one embodiment, the flow velocity at the meniscus (vm) is monitored or sampled. Upon detection of a change in flow velocity at the meniscus (vm), information on this change is fed into the control unit, where it is evaluated. Based on this evaluation, the magnetic flux density is regulated in a suitable way to either maintain the secondary flow pattern or, should it be deemed suitable, change the flow. According to one preferred embodiment, the magnetic flux density is then controlled to maintain or adjust the flow velocity at the meniscus (vm) to be within a predetermined flow velocity range.
According to one alternative embodiment, the upwardly-directed secondary flow (vu) at one of the molds narrow sides is monitored or sampled. Upon detection of a change in this upwardly-directed flow velocity (vu), information on this is fed into the control unit. Based on this evaluation the magnetic flux density is regulated to maintain or adjust the flow velocity of this upwardly-directed flow (vu) or, as the flow at the meniscus (vm) is a function of this upwardly-directed flow, to maintain or adjust the flow at the meniscus (vm) to be within a predetermined flow velocity range. This flow velocity range will vary with casting speed, nozzle geometry, nozzle immersion depth, and when gas is purged, the gas flow, superheat and mold dimensions, but shall for the casting slab using a submerged entry nozzle with side ports and a moderate casting speed normally be held within the range mentioned in the foregoing.
According to one further alternative embodiment, the profile of the meniscus, part of this profile or a parameter characterizing it such as the height (hw), location and/or shape of a standing wave, which is generated in the meniscus by the upwardly-directed secondary flow at one of the molds narrow sides, is supervised or sampled throughout essentially the whole casting. The profile of the meniscus and especially the standing wave is closely dependent on the upwardly-directed flow (vu), as is also, as referred to in the foregoing paragraph, the flow velocity at the meniscus. Therefore can any detected change in the profile such as the height, location or shape of this standing wave be correlated to a flow velocity. Based on such correlation or evaluation the magnetic density is regulated to maintain the standing wave, the flow velocity of the upwardly-directed flow and/or the flow velocity at the meniscus within predetermined limits.
According to one preferred embodiment of the present invention the algorithm, statistical model or data-analysis method used for processing the detected changes also includes parameter values for one or more predetermined parameters out of the following group of parameters;
mold dimensions,
nozzle dimensions and nozzle configuration including the angle of the ports,
dimensions, configuration and position of magnetic poles;
composition of metal cast;
composition of mold powder used.
Such a parameter value is included in the algorithm, statistical model or method for data analysis used to evaluate the determined change to the flow and regulate the magnetic flux density of the magnetic field on-line. The parameter is included as a constant value or if relevant as a time-dependent function, which is assumed to vary in a known way over the casting sequence or as a function of any other casting parameter or flow. Examples of dependent parameters which value can be included in the algorithm, statistical model or method for data-analysis as a function of time or other parameter are;
changes in primary flow due clogging and/or wear of nozzle;
superheat of primary flow, i.e. metal upon entry in the mold;
ferrostatic pressure at nozzle exit.
According to one preferred embodiment of the present invention, one or more out of the following group of parameters is monitored or sampled together with the secondary flow during casting;
superheat of the metal upon entry in mold;
ferrostatic pressure at nozzle exit;
flow velocity of primary flow upon exit from nozzle;
any gas bubbling in mold;
casting speed;
mold powder addition rate;
position of meniscus in mold and relative nozzle port;
position of nozzle port relative mold;
position of magnetic field(s) relative meniscus and nozzle ports;
direction of magnetic field; and
any other casting parameter deemed critical for the secondary flow and which is likely to change during casting.
Preferably one or more these parameters is supervised or sampled throughout essentially the whole casting process and included on-line in the algorithm, statistical model or method for data analysis used to evaluate the determined change to the flow and regulate the magnetic flux density of the magnetic field on-line. The changes can be due to a time-dependent process or be due to an induced change of the casting conditions. These parameters which are accommodated for in the algorithm, statistical model or method for multivariate data-analysis will thereby effect the on-line regulation of the magnetic flux so that the magnetic flux density can be adopted to these changes and a better control of the secondary flow is accomplished.
Preferably the algorithm, numerical model or method for multivariate data analysis used in addition to the monitored or sampled flow parameters also include further casting parameters in the form of preset or predetermined constants, predetermined functions as well as monitored or sampled parameter values. Thus will the controlled secondary flow be more stable and well adopted to give the preferred flow pattern for the conditions actual prevailing in the mold.
According to a further embodiment, the control unit is also associated to one or more further electromagnetic devices, which are arranged to apply one or more alternating magnetic fields to act upon the melt in the mold or in the strand. Such electromagnetic device are stirrers which can be arranged to act on the melt in the mold or on the melt down-streams of the mold, e.g., on the last remaining melt in the so called sump but also high-frequency heaters are used preferably applied to act on the melt adjacent to the meniscus to avoid freezing, melt mold powder and provide good thermal conditions, e.g., when casting with low superheat.
The present invention according provides means to adopt the flow and thereby also thermal conditions to achieve the desired cast structure while ensuring the cleanliness of the cast product and same or improved productivity. The embodiments which include monitoring or sampling of further parameters and/or information on induced changes in production parameters are especially favorably as they provide the possibility to, upon the detection of a change in a casting parameter, adopt the magnetic flux density to counteract any disturbance like to come as a result of this change or take measures to minimize such a disturbance known to be the result of such change.